The classical voltaic cell stores electrical energy between two electrodes, and the energy is stored by electrochemical means. Such a cell is sometimes called a “Faradaic” cell to distinguish it from a classical capacitor which stores electrical energy in an electrostatic fashion. The classical cell is a two-terminal device. In most industrial and automotive applications, a multiplicity of voltaic cells are placed in series, defining a battery. In some applications, the cells are placed in series-parallel combinations, defining a battery. The battery is charged and discharged hundreds or thousands of times during its service life. Each cell has two electrodes, typically composed of non-identical metals or other conductive materials, with an electrolyte disposed between the electrodes.
From time to time it has been proposed to place a third electrode in a voltaic cell. It has been proposed that the third electrode might be used for sensing voltage, so as to arrive at an estimate of some parameter of interest in the cell. See for example P. F. Grieger et al., “Sealed battery with charge-control electrode”, U.S. Pat. No. 3,424,617 A, Jan. 28, 1969, and H. Reber, “Hermetically sealed storage battery including an auxiliary electrode”, U.S. Pat. No. 3462303 A, Aug. 19, 1969.